In recent years, communication technology for mobile terminals such as cell phones and smart phones has shown remarkable progress. The number of users of mobile communication terminals has been increasing every year, and thus the amount of data communication by individual users has been increasing. Accordingly, improvement of efficiency of frequency use and the like are required for base station antennas for mobile communications.
For a base station antenna for such mobile communications, dual-polarized antennas (such antennas as using vertical and horizontal polarizations or +45 degree and −45 degree polarizations) have become the mainstream. Dual-polarized antennas are capable of performing polarization diversity or cross polarization MIMO (Multi-Input Multi-Output).
On the other hand, due to communication traffic shortages in urban areas and the like, antennas for small cells have been increasingly used, which cover an area smaller than the areas having been covered by conventional base station antennas so far (macrocells). Different from macrocell antennas that are usually placed on steel towers or on the rooftop of tall buildings, such small-cell antennas are assumed to be mounted on walls, rooftop, or the like of relatively short buildings. Such small-cell antennas are easily visibly recognized, and thus are desired to be reduced in size and made thinner from the viewpoint of preserving esthetic features of streets, such as consideration of urban landscapes and the like.
For a thin antenna, for example, Patent Document 1 discloses a planar antenna having a thin structure in which multiple CRLH (Composite Right/Left Handed) transmission lines are printed on a dielectric substrate. In Patent Document 1, the feeding phase to each CRLH transmission line can be changed, and it is thereby made possible to easily switch between polarized signals.